Rabbit sperm motility was studied in relation to the morphological character of the head membranes of sperm isolated from the uteri and the isthmus and ampulla of oviducts of does sacrificed at 11/2, 6, 8, 10, and 12 hrs post-coitus. Most spermatozoa isolated from the tubal isthmus are non-motile and their acrosomes are either absent or the acrosomal cap is lifted, whereas motile isthmic sperm have intact head membranes. Motile spermatozoa with visibly altered head membranes were recovered from the ampullae of oviducts at 6 and 8 hours post-coitus, i.e. before ovulation. In a quantitative study of sperm transport, rabbit spermatozoa were recovered at all levels of the female reproductive tract at 1, 15 and 90 minutes post-coitus. Most oviducal sperm recovered at 1 and 15 minutes after mating are non-motile, have altered head membranes and were recovered exclusively from the upper ampulla of the oviduct, the fimbria or the surface of the ovaries. Spermatozoa were not found in the isthmus until 90 minutes post-coitus. Sperm transport in the rabbit can now be divided into two phases: (1) a rapid phase where sperm transported to the oviduct appear non-viable and probably are not involved in fertilization, and (2) a sustained phase of transport during which the isthmus of the oviduct acts as a selective barrier to sperm movement to the lower ampulla-the site of fertilization. BIBLIOGRAPHIC REFERENCES: J. W. Overstreet and G. W. Cooper 1975. Reduced sperm motility in the isthmus of the rabbit oviduct. Nature 258: 718-719. G. W. Cooper and J. M. Bedford 1976. Asymmetry of spermiation and sperm surface charge patterns over the giant acrosome in the musk shrew (Suncus Murinus). J. Cell Biology, in press.